It is well known to provide a passive seat belt system wherein one end of the belt is mounted inboard the seat and the outboard belt end is mounted on the door. A retractor is provided at one of the belt ends so that the belt is automatically wound and unwound between restraining and non-restraining positions by the swinging movement of the door. It is characteristic of such a passive belt system that many layers of belt are wound upon the retractor reel when the door is closed and the belt is in the restraining position. It has been observed in the prior art that the imposition of an occupant restraint load upon the belt subsequent to locking of the reel by a vehicle sensitive inertia locking mechanism causes the belt to tighten or spool-down on the reel and thereby extend the effective length of the restraint belt.
U.S. Pat. No. 4,120,466, issued Oct. 17, 1978 to Adomeit, discloses a self-actuating belt clamping device in which the belt is guided around the end of a lever so that imposition of an occupant restraint load on the belt pivots the lever to clamp the belt between a locking surface carried on the lever and a locking surface mounted on a fixed housing.